Biology Reference
In-Depth Information
Fig. 15
Cell conformation
that minimizes the
combined energy of the
body cytoskeleton and the
boundary.
Bold lines
represent microtubules.
The cell boundary is
triangulated. Reproduced
from Arkhipov and Maly
(
2006a
, doi:10.
1088/1478-
3975/3/3/006) with
permission from IOP
Publishing
parameters describing the individual cell (the number of microtubules and the oncotic
equilibrium volume) taking realistic values, the conformation minimizing the total
energy of the cell body and boundary is asymmetric: The centrosome is off center
within the boundary and the microtubules are bent unequally (Fig.
15
).
This cell boundary model, which takes into account not only the surface area
and tension but also the enclosed volume and oncotic pressure, can also be used
to answer the question of how the intrinsically undefined orientation of the sponta-
neously established asymmetry can be influenced by the cell's environment.
This question arises, for example, in the biology of lymphocytes, whose intrinsi-
cally asymmetric cell body acquires a nonrandom orientation on contact with an
infected cell, directing the secretion of the immune response mediators at this cell
from the lymphocyte's centrosome region. To assess the relative energy of the cell
boundary corresponding to the different orientation of the asymmetric cell body, the
approach taken in the last two sections can be reversed: The shape of the cell body
can be modeled as a constraint for the dynamics of the cell boundary.
Using this approach, Arkhipov and Maly (
2006a
) and Baratt et al. (
2008
) calcu-
lated the energy landscape of all possible orientations of the body in a cell whose
boundary is deformed by the contact with the target surface. The lowest point on
this landscape corresponds to the preferred orientation, which under realistic
assumptions about parameters turns out to be one with the centrosome facing the
contact zone (Fig.
16
). Moreover, multiple minima on the landscape correspond to
subpopulations of cells that are numerically predicted to coexist under the same
conditions (Fig.
17
). These subpopulations, which differ by the centrosome orienta-
tion, find their counterparts among cell populations that prove heterogeneous in
experiments (Baratt et al.
2008
). Future analysis of the systems biomechanics of the
